Overview: The Cell Cycle
Many eukaryotic cells divide through the method of mitosis. However, it is only one part of the cell cycle. A cell cycle is a recurring sequence from the beginning of one cell division all the way through to the beginning of the next mitosis. After mitosis, the cell grows to double its size, with increased enzyme production and chemical activity. Cells that are going to divide go through an additional phase that includes growth of chromosomes. After that part in the cell cycle, the cell goes through a second period of growth. For example, suppose a bean cell takes 19 hours to complete its entire cycle. Roughly two hours will be spent in mitosis, five in the first growth phase, seven in the growth of chromosome phase (the S phase), and five in the second growth phase, before the cycle begins all over again with new cells.
Prophase
During the first phase of mitosis, or prophase, chromosomes first coil up into short rods. Spindle fibers that are made of protein start to align pairs of chromosomes. This stage actually takes most of the time during mitosis. It is usually divided into three separate periods.
Metaphase
During metaphase, the chromosomes are moved around by the spindles until they all line up along the equator of the cell. Each pair of chromosomes is joined at the center by a structure called the centromere, and each centromere has a separate fiber. The chromosomes still appear very thick.
Anaphase
During this phase, the thick chromosomes (called chromatids) are pulled apart by the spindles. Each new half migrates to opposite poles of a very large cell. However, the cell itself has not completely divided.
Telophase and Cytokinesis
After the half chromosomes have moved to opposite poles, the spindle fibers vanish inside the cytoplasm. A new nuclear membrane forms around each set of chromosomes. After the process is complete, the cytoplasm divides. New outer membranes encase both new cells. In plants, the boundaries form cell plates, creating a cell wall between the two cells. In animal cells, the cell membrane completely separates the two cells.
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